Technical Field
This disclosure relates to an imaging unit, an image reading device, and an image forming apparatus.
Description of Related Art
Conventionally, imaging units have been used in digital cameras or scanners and are configured to mount, on the circuit board thereof, image pickup elements such as CCDs and CMOSs and/or ICs. As is known, these imaging units generate heat as energized.
In imaging units, linear expansion coefficients of the image pickup elements and of the circuit board generally differ from each other. For instance, the linear expansion coefficient of the image pickup elements is smaller than that of the circuit board. That is, the expansion amount of the image pickup elements is different from that of the circuit board. Accordingly, when the image pickup elements and the circuit board respectively expand in the different expansion coefficients due to the heat generation or the like, the image pickup elements may be deformed by stress caused by the expansion. As a result, the imaging surface of the imaging unit is deformed or bent, and thus the optical performance of the imaging unit may be deteriorated due to a change of the optical path length caused by the deformation.
To avoid this problem, Patent Literature 1 (Japanese Laid-Open Patent Publication 2011-18747) teaches to provide, on the surface of the circuit board opposite to the surface mounting the image pickup element, a low thermal expansion member having a low thermal expansion coefficient. The low thermal expansion member has a smaller expansion coefficient than the circuit board. By having the low thermal expansion member, the imaging unit can sufficiently absorb the stress caused by the thermal expansion of the circuit board during a heating treatment for mounting the image pickup element on the circuit board. Consequently, the imaging unit of PL1 can suppress a deformation of the image pickup element generated by stress on the circuit board caused by a temperature change.